1. Field of the Invention
This invention relates in general to an improved capacitor bushing seal and more particularly to a method of preparing porcelain for soldering without electroplating.
2. Background
Capacitor bushings serve several functions. The primary function of the bushing is to isolate electrical leads from the metal body of the capacitor tank and from each other. The capacitor bushing is also used to isolate the contents on the inside of the capacitor from the outside environment. It is necessary to protect the internal capacitor components from contact with the exterior environment since contamination by moisture or dust may degrade the dielectric capacitor fluid and lead to capacitor failure. Also, the capacitor bushing seals the dielectric fluid inside and prevents it from escaping to the environment.
A problem is presented in joining the capacitor bushing, which is made of an insulating material such as porcelain, with the capacitor tank, which is usually stainless steel and the terminal cap which usually is a tinned metal such as brass or cooper. A traditional method used in bonding the porcelain bushing to the tank cap and terminal cap has been to solder the bushings to the metal parts.
Soldering the bushings using present technology involves a complicated procedure wherein two coats of a silver-based paint are applied, drying between each coat, fired, and then electroplated. Electroplating is a procedure wherein copper is plated onto the silver paint which has been heat fired into the porcelain. The plating process involves dipping the fired silver plated bushings in a dilute sulfuric acid bath, rinsing twice, then dipping the bushings in a nitric acid bath, rinsing again, dipping the bushings into a copper plating solution consisting of water, sulfuric acid and cupric sulphate and running current through the solution to deposit the copper. After being plated with copper, the bushing is subjected to two water rinses and then is plated with tin in a manner similar to the copper plating. This is followed by two more water rinses, a sodium carbonate rinse, a hot water rinse, drying, and, finally, the soldering step. As may be seen, there are significant problems involved with the traditional electroplating process used to prepare bushings.
The most obvious problems are the complicated, time-consuming, labor intensive number of steps which are involved. Even more significant is the extra costs of materials such as the baths of sulfuric acid, nitric acid, copper-plating and tin-plating solution. Yet another problem is. These baths contain chemicals which are considered environmental contaminants and must be strictly accounted for when disposed of. Also, fumes from the baths require special venting which in itself requires additional construction maintenance costs since the corrosive fumes quickly degrade venting equipment.
Attempts to apply solder to insulating materials are shown in U.S. Pat. No. 4,101,710. However, prior art methods have dealt with providing an electrical solder connection between the thick film composition and other electronic parts and have not dealt with situations where metal and ceramic must be joined in a manner capable of withstanding pressure. See also U.S. Pat. No. 3,649,567 which discloses metallizing compositions applied to glass.